1. Field of the Invention
The present invention relates to a variable-power optical system used for, for example, a video camera or an electronic still camera and an imaging apparatus, and more particularly, to a variable-power optical system suitable to be used for a monitoring camera and an imaging apparatus including the variable-power optical system.
2. Description of the Related Art
Monitoring cameras have been used to operate unmanned facilities. In recent years, particularly, there is an increasing demand for monitoring cameras capable of changing power. A fast optical system having a large aperture ratio needs to be used as a variable-power optical system for the monitoring camera such that it can specify a subject even under low-brightness imaging conditions. In addition, an optical system applied to the monitoring camera needs to have a compact structure and a high optical performance.
For example, JP-A-2006-119574 and JP-A-2007-94174 disclose variable-power optical systems that have a large aperture ratio, a small size, and a high optical performance and can be mounted to the monitoring camera. JP-A-2006-119574 discloses an optical system that includes a first negative lens group, an aperture diaphragm, and a second positive lens group arranged in this order from an object side. In the optical system, the first group includes three single lenses, that is, two negative lenses and one positive lens. JP-A-2007-94174 discloses an optical system that includes a first negative lens group and a second positive lens group arranged in this order from an object side. In the optical system, the second group includes an aperture diaphragm, and a cemented lens of a negative meniscus lens and a positive lens is arranged closest to the object side in the second group.
However, the optical system for the monitoring camera needs to have a wide angle of view in order to monitor a wide range. In addition, in recent years, there is an increasing demand for a monitoring camera capable of obtaining a high-quality image. Therefore, there is an increasing demand for a variable-power optical system capable of corresponding to a camera including an imaging device provided with 1,000,000 pixels or more. However, in the optical system according to the related art, it is difficult to achieve an optical performance capable of corresponding to an increase in the number of pixels while maintaining a large aperture ratio required for the monitoring camera and obtaining a wide angle of view.